Johnny Whitaker, Plato's former manager and a friend of the family, told ABCNews.com he once tried to intervene in Lambert's drug use.
"I did say, 'You know your mother had a problem, which means you may have a problem because it is a family disease,'" said Whitaker, a former child actor who played Jody on "Family Affair" and is now a certified drug counselor. "Dana passed away because of the disease. And when the toxicology reports come in, I'm sure they will show something."
Lambert, 25, committed suicide on May 6, just days before Mother's Day and the 11th anniversary of Plato's 1999 death from a drug overdose.
In the days leading up to Lambert's suicide, a long-time family friend told Whitaker that Lambert had "become very withdrawn and hadn't talked to anybody for three or four days." The same friend told Whitaker that Lambert always said, "he wanted to be with Mom."
The Tulsa, Okla., office of the Chief Medical Examiner confirmed to ABCNews.com that Lambert died last week from a self-inflicted shotgun wound to the head. Toxicology test results are still pending.
Whitaker said Lambert's family once asked him to talk to the young man, an aspiring rapper and singer, about his drug use. At the time Lambert only admitted to using marijuana, Whitaker said.
"I kept the door open and let him know I would be there," Whitaker said. "He wasn't ready to work on it."
Lambert was just 14 when his mother took her own life, overdosing on prescription pills on May 8, 1999. She was 34 when she died.
Plato married Lambert's father, Lanny Lambert. in 1984. Tyler was born a year later. The couple divorced in 1990, and Lanny Lambert was given custody of Tyler.
Even though Lambert mostly grew up with his paternal grandmother, Joni Richardson, he was attached to his mother.
"Dana did absolutely adore her son," Whitaker said.
But he also said Lambert thought he had a closer relatonship with his mother than he really did.
"They didn't," Whitaker said. "Emotionally they did. Physically and actually, they didn't."
Nonetheless, Lambert struggled every Mother's Day.
"Mother's Day was always a difficult time, not only because it was Mother's Day but the anniversary of Dana's death," Whitaker said.
Plato appeared to have a bright future ahead of her when she landed the role of Kimberly Drummond on "Diff'rent Strokes." The show was a hit from its 1978 premiere, and Plato charmed audiences. But she was written off the series in 1984 amid rumors of drug use, and she struggled to find other roles.
In 1989, Plato bared all for Playboy and began starring in B-movies. From there, she dabbled in softcore pornography, including a 1997 feature based on her "Diff'rent Strokes" past.
Meanwhile, legal and substance abuse problems plagued the actress. In 1991 she was arrested attempting to rob a video store at gunpoint, and the following year she was arrested for forging a Valium prescription.
Plato's drug addiction eventually took her life -- she died after overdosing on Vanadom and Vicodin.